Automotive turn signals, in commercial usage, have most commonly employed a flasher unit or stage of the hot-wire type. This type of flasher unit probably is the predominant type in current usage. The conventional automotive turn signal circuit comprises a turn signal switch which is manually actuated to selectively energize the right-hand or left-hand turn signal lamps from the battery through the circuit of the flasher. The flasher, whatever its form, is operative to intermittently connect the turn signal lamps across the battery.
The conventional automotive turn signal circuit includes a series connection of the turn signal switch, flasher contacts or switch, and the selected lamps across the battery. In this conventional circuit the turn signal switch is electrically connected between the flasher switch and the lamps. This order of arrangement, whatever the reasons, has become standardized and, as a practical matter, is of significance in developing new flasher circuits, as will appear subsequently.
As mentioned above, the hot-wire flasher was used almost universally for a period of time; in more recent years, electronic flashers using oscillator controlled flasher switches have come into use. A large number of oscillator controlled flasher circuits have been devised in the prior art, most of them employing transistorized oscillators which control a flasher switch in the form of either a power transistor or a flasher relay. In order for an oscillataor controlled flasher to be useful in the conventional automotive turn signal circuit it must start oscillation when the turn signal switch is closed and stop oscillation when it is opened. The difficulty with this requirement arises from the fact that the order of arrangement of the flasher switch and turn signal switch in the conventional turn signal circuit precludes the straightforward use of the voltage at the turn signal switch from being used as a turn-on and turn-off signal for the oscillator. With the turn signal switch in its conventional location between the flasher switch and load, the voltage at the turn signal switch is intermittently high and low in synchronism with the closing and opening of the flasher switch so long as the turn signal switch is closed. Thus, the turn signal switch produces an ambiguous signal voltage rather than a signal voltage which unequivocally signifies whether the turn signal switch is open or closed, such as a low voltage when the turn signal switch is open and a high voltage when it is closed as would be the case if the turn signal switch and flasher switch are positionally interchanged in the circuit.
The hot-wire flasher which has had such widespread use is basically a two-terminal device, i.e., it has one terminal for connection to the supply voltage and another terminal for connection to the load, the return circuit to the supply voltage source being through the load itself. (Some flashers include a third terminal for connection of a pilot lamp, such auxiliary terminals not being counted in the basic requirement.) The conventional turn signal circuit thus is adapted to receive a flasher by a two-terminal connection.
The problem, stated simply, of adapting an oscillator controlled flasher to the conventional turn signal circuit is that of connecting the flasher into the circuit with two terminals and turning the flasher on or off by closing and opening the turn signal switch. A solution would allow an oscillator controlled flasher to be substituted directly for a hot-wire flasher without any circuit changes; further, the oscillator would be off except when the turn signal switch is actuated, thereby eliminating the mechanical cycling of the relay that would cause premature wear.
The prior art includes several different oscillator controlled flashers. One such flasher is disclosed in the Grontkowski U.S. Pat. No. 3,002,127 wherein a multivibrator is used for periodically actuating a flasher relay. In the system of this patent the multivibrator is started by closing the turn signal switch, with the supply voltage being connected through the turn signal switch and the flasher relay contacts; to keep the multivibrator running when the flasher relay contacts open, an auxiliary contact is provided on the flasher relay along with steering diodes to provide supply voltage to the multivibrator.
Another prior art multivibrator controlled flasher is disclosed in the Domann et al U.S. Pat. No. 3,329,868. In this patented flasher the multivibrator is held in an energized but non-oscillating state even when the turn signal switch is in the neutral or open position. When the turn signal switch is closed a separate transistor stage is turned on and enables the multivibrator to start oscillation.
There are other examples of multivibrator controlled flashers which are provided with some form of oscillator on and off control by means of a turn signal switch. The Ivec U.S. Pat. No. 3,478,248 discloses a multivibrator controlled flasher in which the output stage of the multivibrator has a DC path only through a turn signal switch and hence oscillations cannot start until the switch is closed. In the Roberts U.S. Pat. No. 3,576,444 the output stage of the multivibrator receives initial base drive upon closure of the turn signal switch and then oscillation is sustained by subsequent supply of base drive from a storage capacitor. In the Schorter U.S. Pat. No. 3,824,542 the starting and sustaining of oscillations in the multivibrator by closure of the turn signal switch is provided by a special relay having voltage and current coils, together with a special capacitor and diode circuit.
The subject invention provides a multivibrator controlled flasher circuit especially adapted for a conventional automotive turn signal system. The flasher is turned off and on by the turn signal switch and is basically a two-terminal device so that it can be used in the same circuit as a hot-wire flasher without requiring circuit change. Additionally, the inventive circuit may be implemented with standard integrated circuits and constructed as a plug-in module.
The invention comprises a free-running multivibrator adapted to oscillate when an enabling signal is applied thereto; an enabling means applies the enabling signal only when the voltage at the multivibrator output and the voltage at the turn signal switch are in a predetermined state relative to each other. The multivibrator comprises first and second logic gates and the enabling means is also a logic gate. The gate of the enabling means has one input connected with the multivibrator output and another input connected with the turn signal switch through a logic voltage circuit. The logic gates may be either NAND gates or NOR gates and the invention is useful in either a negative or positive ground automotive electrical system.